Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Abstract

Long‐term trends in middle atmosphere (~40–100 km; here extended up to 110 km), 15 μm carbon dioxide (CO2) cooling rates, and their responses to solar cycle (SC), quasi‐biennial oscillation, and El Niño‐Southern Oscillation are investigated over Indian tropical region (10°N–15°N) using TIMED‐SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry) observations during 2002–2015 (23–24 SCs). The CO2 cooling directly depends on temperature so that higher temperature causes more infrared emission from CO2 at 15 μm band and thus provides larger CO2 cooling. The 15 μm CO2 cooling trends are relatively smaller below ~70–80 km, and their amplitude increases above this height. The trends are positive between ~40 and ~62 km peaking at ~47 km (0.06 ± 0.014 K/day/decade) and negative at ~63–74 and ~79–84 km. Further the positive trends increase sharply above ~84 km at the rate of ~0.1–3.0 K/day/decade between 85 and 110 km. The CO2 cooling response to SC, El Niño‐Southern Oscillation, and quasi‐biennial oscillation is smaller below ~70–75 and stronger above this height. The 15 μm CO2 heating exhibits semiannual variation with ~0.1–0.8 K/day during equinoxes between ~70 and 77 km. In addition, the increase in CO2 cooling significantly influences the background temperature and thereby ozone concentration, neutral number density and layer thickness (width) in the stratosphere and mesosphere.